6The Ts65Dn mouse model of Down syndrome has demonstrated as behavioral phenotype consistent with aspects of the human phenotype of DS. The aims of this proposal are first to continue to determine the behavioral phenotype of this interesting model and second to begin to determine the mechanisms by which over-expressed genes produce the neural and behavioral phenotype. Because DS is a complex disorder, no one brain region is responsible for the cognitive phenotype. Similarly, in this Ts65Dn model, multiple brain regions are implicated by the findings to date, namely the prefrontal cortex, hippocampus, cerebellum and perhaps amygdala. Understanding how deficits in these multiple regions interact to produce the DS phenotype will require a thorough exploration of the behavioral phenotype. We will use contextual and cued fear conditioning, skilled reaching, chaining behavior, reference and working memory, delayed and alternation and a range of eyeblink conditioning paradigms. Our approaches in this aim have from the start been guided by the research and theorizing of our colleagues (Pennington and Nadel) on Project 8. In the new grant period, we have designed experiments in parallel, and in future will be able to make use of their findings with humans to design tests for the animal models. In addition, hypothesis about neural systems involved in DS that derive from the human work can be tested in animals. The hypothesis to be tested regarding neural involvement in DS is that basal forebrain and medial septal cholinergic neurons are involved in the prefrontal and hippocampal deficits in this mouse model. This will be testing by manipulating these cholinergic systems with a well-established prenatal choline supplementation and examining number and size of cholinergic neurons and dendritic spins on hippocampal and cortical neurons. The role of the over production of interferon receptors in the cholinergic phenotype will be tested by crossing the Ts65Dn mice with knockout mice for these receptors to normalize their copy number. In addition, we will make use of transgenic and knockout mice produced by other projects that can be used to determine the role of other individual genes and gene regions in the cognitive phenotype of DS.